1. Field of the Application
This invention is in the field of laser based defect localization analysis of integrated circuits, and, more specifically, in the field of design debug and/or failure analysis of integrated circuits using laser illumination.
2. Related Art
Laser Assisted Device Alteration (LADA) is a technique employed in the test and debug stage of chip design. It is specifically used for isolating critical performance limiting nodes in complex, defect-free integrated circuits. It has also found wide use in localizing process defects as the LADA effect easily modulates transistor characteristics in the same path as the process defect.
The LADA technique is based on the ability of a continuous wave (CW) laser to generate localized photocurrents in an integrated circuit's backside, and thus change the pass/fail outcome of a test stimulus on a “sensitive” transistor and identify the location of faults. The laser is typically of a short wavelength variety on the order of 1064 nm, so that the photon energy is above the silicon indirect band gap (about 1107 nm). This photon energy is required in order to initiate the single photon effect that is used to change the transistor's behavior. Due to the limitation on selection of wavelength of the laser, the current spatial resolution of the localized fault is about 240 nm.
FIG. 1 illustrates a conventional LADA system, which uses a continuous wave laser to induce single-photon electron-hole pairs in the device under test (DUT) from the backside of the chip. A DUT 110 is coupled to a tester 115, e.g., a conventional Automated Testing Equipment (ATE), which is connected to computer 150. The ATE is used in a conventional manner to stimulate the DUT with test vectors and study the response of the DUT to the test vectors. The response of the DUT to the test vectors can be further investigated using the LADA. For example, if the DUT fails a certain test, LADA can be used to investigate whether the DUT can pass under certain conditions and, if so, which device, i.e., transistor, was responsible for the failure. Conversely, when the DUT passes certain tests, LADA can be used to investigate under which conditions the DUT will fail these tests and, if so, which device, i.e., transistor, was responsible for the failure.
The LADA system operates as follows. Tiltable mirrors 130 and 135 and objective lens 140 are used to focus and scan a beam from CW laser 120 onto the DUT 110. This allows the laser 120 to generate photo carriers in the silicon of the DUT without resulting in localized heating of the device. The electron-hole pairs so generated affect the timing of the nearby transistor, i.e., decreasing or increasing transistor switching time. The tester is configured to place the operating point of the device under test in a marginal state by applying a recurrent test loop of selected voltage and frequency. The laser stimulation is then used to change the outcome of the tester's pass/fail status. The beam's location at each point is correlated to the pass/fail outcome of the tester, so that when a change is detected, i.e., a previously passing transistor is now failing or vice versa, the coordinates of the laser beam at that time points to the location of the “borderline” transistor.
The present state-of-the-art in laser assisted fault localization is of about 240 nm resolution. The limitation on further improvement of the single photon LADA resolution is imposed by the laser light wavelength. Optical absorption of silicon at smaller than 1064 nm becomes the major obstacle for delivering light to the transistor through the backside. However due to the continued scaling of chip designs, higher resolution is required in order to provide fault localization in state of the art chips. For example, at 22 nm design rule it is doubtful that conventional LADA will be able to resolve among four neighboring transistors.
Optical beam induced current (OBIC) is another test and debug analysis in which laser beam is illuminating the DUT. However, unlike LADA, OBIC is a static test, meaning no stimulus signal is applied to the DUT. Instead, the laser beam is used to induce current in the DUT, which is then measured using low-noise, high-gain voltage or current amplifiers. OBIC has been used in a single-photon mode and in a two-photon absorption mode, sometimes referred to as TOBIC or 2P-OBIC (two-photon optical beam induced current).
Two-photon absorption (TPA) is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from one state (usually the ground state) to a higher energy electronic state. The wavelength is chosen such that the sum of the photon energy of two photons arriving at the same time is equal to the energy difference between the involved lower and upper states of the molecule. Two-photon absorption is a second-order process, several orders of magnitude weaker than linear (single-photon) absorption. It differs from linear absorption in that the strength of absorption depends on the square of the light intensity, thus it is a nonlinear optical process.
FIG. 3 illustrates a comparison of a single-photon absorption on the left and two-photon absorption on the right. Since the absorption of two-photon is a second order process, it enables a 1.4 (2) times higher resolution than single-photon absorption. However, it requires a higher peak power, pulsed beam, so that the probability of two photons arriving at exactly the same time is drastically increased. Therefore, femtosecond laser pulses, with pulse width of about 100 fs, have been used in the art to generate two-photon absorption.